Mammalian cells cultured in vitro will be used to investigate the relative biological effectiveness (RBE) of the low energy emission from Iodine-125 and Cesium-131, at low dose-rates. Our previous investigations involving Iodine-125 at the very low dose-rates characteristic of permanent implants (circa 8 rad/hour) will be extended to the somewhat higher dose-rates likely to be used in removable, temporary implants. The studies will include the newly available radionuclide Cesium-131; dosimetric, microdosimetric and radiobiological measurements will be made. The RBE determinations performed to date with an established line of hamster cells (CHO) will be repeated with normal diploid human fibroblasts which exhibit a survival curve for acute x-ray exposures characterized by a small shoulder. A variety of human cell lines of normal and malignant origin will be used to investigate the relative importance of the repair of sublethal and potentially lethal damage, with reference to the response of the cells to protracted exposures at low dose-rate.